The present invention relates to a receiver for receiving radio-frequency pseudo-random encoded signals from satellites of a universal ranging system.
A positioning or timing receiver in a universal ranging system must receive and process signals from several satellites, such as those of the GPS or Glonass constellations, to obtain a measurement of its position or to establish a timing reference. The signals from a given satellite are processed in a dedicated channel, a conventional example of which is shown schematically in FIG. 1.
Referring to FIG. 1, a receiver channel 10 of a positioning receiver comprises generally an analogue section 11, a digital section 12 and a digital/software interface 13. The interface 13 is connected to a micro-processor (not shown) which processes interface output signals with software. The primary signal path of the analogue section comprises, from an antenna 14, a radio frequency (RF) amplifier 15, a first mixer 16, a first intermediate frequency (IF) amplifier 17, a second mixer 18, a second IF amplifier 19 and an analogue to digital converter (ADC) 20. A 20 MHz reference frequency oscillator 21 provides a 20 MHz reference signal to a phase lock loop (PLL) 22, which provides local oscillator signals on outputs 23 and 24 to the mixers 16 and 18 respectively.
The ADC 20 provides a three-level output signal (i.e. occupying two bit lines) sampled at 15.42 MHz to signal inputs of each of two digital mixers 25 and 26, which define an input of the digital section 12. The oscillator 21 is connected to feed the 20 MHz reference 30 frequency signal also to a digital frequency divider 27, which provide orthogonal phase digital cos and sin signals at 5 MHz to local oscillator inputs of the mixers 25 and 26 respectively. In-phase (I) and quadrature (Q) digital signals are therefore provided on mixer outputs 28 and 29 respectively. A carrier numerically controlled oscillator (NCO) 32 is connected to receive the 20 MHz signal from the oscillator 21. The carrier NCO 32 is controlled to provide oscillator signals on an output 33 at such a frequency as to cause the mixers 30 and 31 to provide baseband I and Q signals on their outputs 34 and 35 respectively. These baseband signals are modulated only (in the case of signals from GPS satellites) by the C/A code at 1.023 MHz and by the data which is carried on the signals at 50 bits per second.
A code NCO 36 is connected to receive the 20 MHz signal from the oscillator 21 and is controlled to provide code clock signals at an output 37. The code clock signals are referenced to the frequency of the oscillator 21 but are equal in frequency to the code, (1.023 MHz for GPS L1 signal codes). During code tracking, a code replica generator 38 receives the code clock signals and is controlled to provide prompt code replica signals on an output 39 and early-minus-late code replica signals on an output 40. The early-minus-late code replica signals are generated by subtracting code replica signals which are phase-delayed with respect to the prompt code replica signals from code replica signals which are phase-advanced with respect to the prompt signals. First and second prompt code mixers 41 and 42 are connected to mix the baseband I and Q signals on the outputs 34 and 35 with the prompt code replica signals to provide prompt I and prompt Q signals on respective outputs 43 and 44. First and second early-minus-late mixers 45 and 46 similarly provide early-minus-late I and early-minus-late Q signals on respective outputs 47 and 48 by mixing the baseband I and Q signals with the early-minus-late code replica signals. Each of the signals provided by the mixers 41, 42, 45 and 46 is accumulated in a respective accumulator 49-52. The accumulators 49-52 are clocked by the oscillator 21, and subsequently buffered into data form by respective buffers 53-56. The buffers 53-56 are clocked by a signal obtained from the oscillator 21 by a frequency divider 57. The output signal of the frequency divider 57 thereby defines accumulation intervals. The outputs of the buffers 53-56 are collated into an output 58 for subsequent software processing. A feedback path (not shown) allows the frequency and phase of both the carrier NCO 22 and the code NCO 36 to be dynamically controlled to maintain alignment with the 30 received signal.
In another known positioning receiver, code replicas may be switched to vary the delay of the early-minus-late code signals so that the receiver channel can function as a conventional correlator during signal acquisition and as a narrow correlator during signal tracking. In either case, a prompt code replica is aligned with the C/A code modulated onto the received signal when an output signal of an early-minus-late correlator is zero.
In a multipath environment, signals which are reflected before arrival at the receiver are delayed in time with respect to the arrival of the direct signals. Although reflected signals are of lower amplitude than the direct signals (except when the line of sight is obstructed), they cause problems with alignment of the prompt correlator with the early-minus-late correlator, when the reflected signals are within around one period or chip of the code of the direct signals. This is a recognised problem which is addressed at least in part by the narrow correlator operation mentioned above. Narrow correlators have the effect of reducing the effect that a reflected signal has on the alignment of the correlators by reducing the effect of the reflected signals. Two other approaches have been taken in an attempt to improve signal resolution in the presence of multipath signals.
Firstly, it is known to use in a receiver a plurality, typically 4 or 6, of parallel correlators each using a different delay of the early-minus-late code signals. This approach, in effect, involves sampling the discrimination pattern at a number of points, equal to the number of correlators, to provide data which can be resolved by software as a series of simultaneous equations. The solutions to the simultaneous equations identify the reflected signals, which can thus be isolated from the direct signal. Obviously, a greater number of correlators results in a greater resolution of the signals and, therefore, more accurate position measurements. However, such an approach requires a considerable increase in hardware (the extra correlators) and in processing power to resolve the signals. Although a similar effect can be achieved by assigning the use of plural channels for the resolution of one signal, each channel having a differently phased early-minus late code signal, the same disadvantages are present.
A second approach has been to generate code signals comprising a set of recurring non-zero three-level gating signals having equal positive and negative areas and a polarity at a centre point which depends on whether a corresponding edge of the code modulated on the received signals is a rising or a falling edge. This approach provides an error signal or discrimination pattern which allows steering around the alignment point but which has zero response to multipath signals falling more than a short distance from the alignment point.
According to the present invention, a receiver, for receiving and processing radio-frequency pseudo-random encoded signals, comprises in a channel thereof:
a code replica generator arranged to provide code replica signals on an output thereof;
a mask generator device controllable to provide any one of at least two predetermined time varying mask signals at an output thereof;
a frequency translator arranged to receive and to frequency translate the encoded signals and to provide translated signals in response thereto;
a signal path of the channel comprising between the frequency translator and an accumulator:
a code mixer arranged to mix the translated signals with the code replica signals; and
a mask mixer arranged to mix the translated signals with the mask signals, the mask signals having a period equal to an integer multiple of the chip period of the code replica signals.
The controllability of the mask generator device gives the receiver versatility in terms of the correlations that can be performed. Significantly, it is possible to construct a receiver which has additional functionality without the use of additional clock frequencies, clock trees or other clocking circuitry. This is especially advantageous in deep, sub-micron design, where wiring delays of ten exceed gate delays, and if a design needs to be reused. A clock-based delay mechanism may also restrict a receiver design to a particular frequency plan and therefore limit the satellite constellations from which signals can be used to make position measurements.
According to a second aspect of this invention, a method of processing radio-frequency pseudo-random encoded signals comprises: frequency translating the encoded signals; mixing the frequency translated signals with code replica signals; storing at least two different mask patterns; providing mask signals corresponding to one of the mask patterns and having a period equal to an integer multiple of the chip period of the code replica signals; mixing the mask signals with the frequency translated signals; and accumulating the resultant signal.
According to a third aspect of this invention, a receiver for receiving radio frequency pseudo-random coded signals, a channel of the receiver comprises:
a frequency translator arranged to frequency translate the coded signals to provide translated signals;
a first mixer arranged to mix the translated signals with a local oscillator signal to provide in-phase translated signals on a first signal path;
a second mixer arranged to mix the translated signals with a quadrature version of the local oscillator signal to provide quadrature translated signals on a second signal path;
each of the first and second signal paths comprising a respective further mixer subsequent to its respective mixer, and a respective accumulator subsequent to its at least one further mixer;
characterised in having a switch connected subsequent the first and second mixers and prior to the further mixer in each path, the switch being controllable to provide the at least one further mixers in both of the first and second paths with one of the in-phase translated signals and the quadrature translated signals, the signals on the first and second signal paths each being mixed with an early-minus-late version of a replica code.
In this way it is possible to construct a receiver having a channel which, after code lock, has two signal paths provided with the same signal, which allows two different techniques of signal processing to be carried out simultaneously on a signal without the requirement of an extra signal path and associated hardware.
According to a fourth aspect of this invention, a method of detecting the presence of multipath interference on a received signal, comprises:
on a first signal path, mixing the received signal with a first signal;
in a first accumulator accumulating the signal provided by the first signal path;
on a second signal path, mixing the received signal with a second signal different from the first signal;
in a second accumulator, accumulating the signal provided by the second signal path;
comparing an output of the first accumulator with an output of the second accumulator; and
providing an indication if the outputs are not substantially in an expected ratio.